This invention relates to controls for A.C. motors used with standard substantially constant frequency and substantially constant voltage A.C. power sources, and deals more particularly with such control whereby a high average value of voltage is supplied to the motor during starting and whereby, as an energy saving feature, the average value of the voltage supplied the motor is reduced following starting and during the motor's normal running phase.
The control system of this invention may be used with various different types of A.C. motors adapted for use with standard substantially constant frequency and substantially constant voltage A.C. power sources, but is particularly well-adapted for use with induction motors. In such motors, the maximum power requirement usually occurs during starting, and they are therefore, of course, designed to produce the torque necessary for assured starting even though the number of winding turns and other factors required to achieve proper starting are not fully required for running. As a result, unnecessarily high currents flow through the motor windings during running and produce I.sup.2 R heat which not only represents the loss of energy but also may cause heat damage to the motor or its surroundings.
Prior U.S. Pat. No. 4,052,648 shows a motor control system wherein a generally higher average voltage is supplied to the motor during a starting period than during the following running phase. In the control of this patent, however, the power factor of the power supplied to the motor is determined through a relatively complicated circuit and the voltage supplied the motor is controlled to maintain the detected power factor at a commanded value. The general object of this invention is to achieve the asserted power saving benefits of the patented controller with a control which is much less complicated and costly and more reliable, versatile and easy to install.
Other objects and advantages of the invention will be apparent from the following detailed description of the preferred embodiments thereof.